Reinforced long porcelain bushing



Nov. 17, 1970 I KOJI KIKUCHI ET AL REINFORCED LONG PORCELAIN BUSHING Filed June ,5, 1968 FIG -I 4 Sheets-Sheet 1 INVENTORS key Kikuchl K ol'chl Ishizu. Hnw5hi$uzuki fro/uki Khnurl ATTORNEYS Nov. 17, 1970 K0." KIKUCHI .ET L 3,541,220

REINFORCED LONG PORCELAIN BUSHING Filed June 5, 1968 4 Sheets-Sheet z FIG-2 I INVENTORS K0j| Kikuchf Kon'ohi I'shizu Hl'rashi ju uk,"

me w

ATTORNEYS Nov. 17, 1970 KOJI KlKUCHl EI'AL 3,541,220

REINFORCED LONG PORCELAIN BUSHING FIG-3 Filed June 5, 1968 4 Sheets-Sheet 3 [ma 110291? femfi-farea aclg'r Faz'rzf 22 24 INVENTORS KOJ'I. Ka kuchl Hit-05b Suzuk" BY Hir yuki Kim ura ATTORNEYS Nov. 17, 1970 Filed June 3, 1968 FIG-4 4 Sheets-Sht 4.

INVENTORS Hirashi Suzuki Ha'r'oyqki Kimara ATTORNEY 5 United States Patent Office 3,541,220 REINFORCED LONG PORCELAIN BUSHING Koji Kikuchi, Yokohama, Koichi Ishizu, Ichikawa, and Hirosln' Suzuki and Hiroyuki Kimura, Yokohama, Japan, assignors to The Furukawa Electric Company Limited, Tokyo, Japan Filed June 3, 1968, Ser. No. 733,939 Claims priority, application Japan, Sept. 8, 1967, 42/ 76,465; Apr. 19, 1968, 43/26,224 Int. Cl. H011) 17/26 U.S. Cl. 174-12 4 Claims ABSTRACT OF THE DISCLOSURE A reinforced long porcelain bushing having an insulating cylinder of fibre-reinforced plastic fitted in the long porcelain bushing for reducing the internal pressure against the long porcelain bushing. The long porcelain bushing and the insulating cylinder are provided with stable slide seals along a terminal rod to permit external expansion and contraction of the long porcelain bushing and the insulating cylinder. The long porcelain bushing is subjected to a compressive stress to make it strong enough to withstand external force. The insulating cylinder is given a taper such that the inner diameter of the long porcelain bushing can be made smaller. Those portions of the flange of the insulating cylinder which occlude voids and gaps are coated with electric conductive or semi-conductive paint for increasing the partial discharge inception voltage.

The present invention relates to a reinforced long porcelain bushing and more particularly to a reinforced long porcelain bushing for use in extra-high voltage transmission lines, having an insulating cylinder of glass fibrereinforced plastic provided in it to increase its mechanical strength.

Along with the recent sharp increase in transmission line voltage, the porcelain bushing for use for cable pothead or transformer is now required to be much larger in length and diameter to ensure its electrical safety.

For example, a porcelain bushing for use in 400-500 kv. extra-high voltage, transmission line must have a length of the order of 8-10 In. and an inside diameter of the order of 0.4-0.8 in.

From the mechanical point of view, such porcelain bushing is required to withstand an internal oil or gas pressure of 100200 kg./cm at the time of transient pressure rise (due to electrical faults) and an external force such as 1ts own welght, Wll'ld pressure, earthquake, etc.

However, it is very difficult to satisfy the above-mentioned electrical and mechanical requirements by the porcelain bushing only.

For example, in the case of a long porcelain bushing, 10 m. in length and 0.7 m. in inner diameter, made of porcelain having a tensile strength of 250 kg./cm. and a specific gravity of 2.5, even when its thickness is made as large as 0.1 In, its breaking pressure P (the internal pressure at which the porcelain bushing breaks) is 2 X (thickness) (Inner diameter) 1 X (tensile strength) 3,541,220 Patented Nov. 17, 1970 top. Provided that the tensile stress is now represented y 1,

(Length) X (radius) [Weight (Geometric moment of areaJl 2 (Top static load 1 The top static load means a static load that works on the top of the porcelain bushing.

That is, 0 kg./cm. If the external force such as the wind pressure, earthquake, etc., is taken as twice the tensile stress 0' under the static load, then the safety factor (S) of the above- This is less than 2 and indicates that the long porcelain bushing may be broken by comparatively small shocks.

To make a long porcelain bushing strong enough to withstand the above-mentioned excessive internal and external forces, 0.1 m. or so is not a sufficient Wall thickness for it. The thickness must be much larger, which in turn makes the outer diameter of the porcelain bushing extremely larger. Such porcelain bushing can hardly be manufactured in consideration of manufacturing technique and equipment involved.

An object of the present invention is to obviate the above-mentioned disadvantages and provide a long porcelain bushing that has an excellent mechanical strength with a comparatively small wall thickness.

The first feature of the invention is that an insulating cylinder made of fibre reinforced plastic is fitted into a long porcelain bushing to bear the pressure of insulating oil or gas in a cable line, the space between the bushing and the insulating cylinder being filled with low-pressure insulating oil or gas to reduce the pressure against the bushing and that a slide packing is provided between an upper metal fitting secured to an upper flange of the insulating cylinder and a terminal rod, so as to permit free thermal expansion and contraction of the insulating cylinder while keeping the insulating cylinder completely sealed at all times.

The second feature of the invention is that a tensile stress is applied to the insulating cylinder in the long porcelain bushing to give the bushing a suitable compressive stress and increase its bending strength, that is, its mechanical strength against excessive external forces such as wind pressure, earthquake, etc.

The third feature of the invention is that a corona (partial discharge) shielding conductive or semi-conductive paint is coated on the upper and lower flanged portions of the fibre-reinforced plastic insulating cylinder in the long porcelain bushing to increase partial discharge inception voltage in voids present in the flanged portions.

The last feature of the invention is that the insulating cylinder in the long porcelain bushing may be made in a tapered form with its upper flanged portion made smaller in diameter than its lower flanged portion, said larger lower flanged portion being located below the lower end of said long porcelain bushing, thereby making it possible to reduce the inner diameter of the bushing and increase the mechanical strength of the insulating cylinder.

The invention will be further explained in detail in the following examples with reference to the accompanying drawings.

FIG. 1 is a partly sectional side elevation of the first embodiment of a reinforced long porcelain bushing according to the invention.

FIG. 2 is an enlarged partly sectional side elevation of the second embodiment, in which the upper portion of the long porcelain bushing and the upper flanged portion of the insulating cylinder are shown. 7

FIG. 3 is an enlarged sectional view of the third embodiment, in which the upper portion of the long porcelain bushing and the upper flanged portion of the insulating cylinder are shown.

FIG. 4 is a partly sectional side elevation of the fourth embodiment.

Referring to FIG. 1, 1 is a long porcelain bushing Whose upper and lower ends are hermetically connected through metal fittings 2, 2 secured thereto to an upper metal fitting 3 and a lower metal fitting 4, respectively.

Between the long porcelain bushing 1 and each of the upper and lower metal fittings 3 and 4 is inserted a gasket 5 which ensures hermetical seal between these two portions.

According to the invention, an insulating cylinder 6 made of fibre-reinforced plastic and having excellent mechanical strength and electrical strength is fitted in the long porcelain bushing 1. This insulating cylinder 6 comprises a substrate of glass fibre or plastic fibre in roving or crossed state, impregnated with a thermosetting synthetic resin.

This insulating cylinder made of fibre-reinforced plastic 6 has flanges of the same fibre-reinforced plastic 7 and 8 at the upper and lower ends. The upper flange 7 is hermetically connected through a gasket 9 to a metal fitting 10, while the lower flange 8 is hermetically connected through a gasket 9 to the metal fitting 4. The upper metal fitting 10 is held hermetically in contact with terminal rod 12 by the medium of slide packing 11 such as a Y or V ring packing available in market. The slide packing 11 serves to hermetically seal the space between the insulating cylinder 6 and the terminal rod 12 even when the insulating cylinder 6 is subjected to thermal expansion and contraction.

Similarly, the space between the upper metal fitting 3 secured to the long porcelain bushing 1 and the terminal rod 12 is hermetically sealed by slide packing 13, thus permitting the thermal expansion and contraction of the long porcelain bushing 1.

14 is metal adapter for clamping the slide packings 11 and 13. 15 shows an oil feeding passage through which is fed an insulating oil, insulating compound, or insulating gas into a chamber R formed between the long porcelain bushing 1 and the insulating cylinder 6. A chamber R formed between the insulating cylinder 6 and a cable terminal cone 28 is filled up with an insulating oil or gas under the same pressure as the pressure inside the cable line. The pressure of the insulating oil or insulating gas fed into the chamber R is made lower than the pressure of the insulating oil or gas inside the chamber R As explained hereinbefore, the invention makes use of the insulating cylinder 6 of fibre-reinforced plastic fitted into the long porcelain bushing 1 and hence provides the important advantage that the insulating cylinder 6 bears the internal pressure against the long porcelain bushing 1 at the time of transient pressure rise and prevents the long porcelain bushing 1 from receiving any excessive internal pressure; therefore the thickness of the long porcelain bushing 1 need not be made excessively large.

The above-mentioned construction according to the invention that the insulating cylinder 6 made of fibrereinforced plastic is fitted into the porcelain bushing 1, that the flanges 7 and 8 made integral with the insulating cylinder 6 are secured to the upper and lower metal fittings 10 and 4, respectively, and that the slide packing 11, which permits the thermal expansion and contracttion of the insulating cylinder 6, is inserted between the upper metal fitting 10 and the terminal rod 12, insures proper hermetical seal of the chamber R formed between the insulating cylinder 6 and the cable terminal cone 28 even when the insulating cylinder 6 is subjected to thermal expansion and contraction. Such a construction is far superior to the means of hermetically sealing the space between the outer peripheral surface of the in sulating cylinder 6 and the upper metal fitting 3 of the long porcelain bushing 1 with a packing inserted there between. In the latter means of providing a slide seal on the outer peripheral surface of the insulating cylinder 6, the outer periphearl surface of the insulating cylinder 6 is held directly through a slide packing by the upper metal fitting 3 of the long porcelain bushing 1. In this case the outer peripheral surface of the fibre-reinforced plastic insulating cylinder 6 must be finished with high-precision, which is technically difiicult. Such high-precision machine finishing, if ever tried, will deteriorate the inherent property of the insulating cylinder 6, increase its moisture absorption, and reduce its resistance to arc.

As explained hereinbefore, since according to the invention, the slide packing 11 is inserted between the upper metal fitting 10 and the terminal rod 12 extending through it, only the terminal rod 12 needs a high-precision finish to ensure stable seal of the insulating cylinder.

Provided that an insulating cylinder 6 having an inner diameter of 50 cm. and a thickness of 1 cm. is made of plastic reinforced with glass fibre having a tensile strength of 5,300 kg./cm. its breaking pressure P is determined by the above-mentioned Formula (1) as follows:

This indicates that the insulating cylinder 6 whose thickness is as small as 1 cm. can withstand a pressure of -200 kg./cm. at the time of transient pressure rise, when the thickness of the insulating cylinder 6 is increased to 2 cm., its breaking pressure becomes 420 kg./cm. and hence its breaking strength is considerably increased.

The above-mentioned first embodiment of the reinforced long porcelain bushing according to the invention has an insulating cylinder of fibre-reinforced plastic fitted into it to reduce the internal pressure against it during normal operation or and at the time of transient pressure rise and obtain a stable slide seal.

The second embodiment of the reinforced long porcelain bushing according to the invention is such that a compressive stress is constantly given the long procelain bushing to improve its bending strength and make it strong enough to Withstand external force such as wind pressure, earthquake, etc.

In the embodiment shown in FIG. 2 the insulating cylinder 6 of fibre-reinforced plastic is fitted into the long porcelain bushing 1 and made integral at each end with the upper flange 7 and the lower flange (not shown). The lower flange is secured to the lower metal fitting and the upper flange 7 to the upper metal fitting 10 which is hermetically held through the slide packing 11 by the terminal rod 12. The above construction is the same as that of the first embodiment shown in FIG. 1. In the embodiment shown in FIG. 2, however, a bolt 17 passing through an upper metal fitting 16 secured to the long porcelain bushing 1 is fixed in the upper metal fitting 10 secured to the upper flange 7 of the insulating cylinder 6. A spring 18 is attached to the bolt 17 between its head and the upper metal fitting 16 and compressed by a clamping member such as a nut 19 to apply tensile stress to the insulating cylinder 6 and compressive stress to the long porcelain bushing 1.

The clamping member 19 is so turned as to compress the spring 18 hard enough to cause a necessary compressive stress to the long porcelain bushing 1, regardless of how the distance between the upper metal fittings of long porcelain bushing 1 and the insulating cylinder 6 may be changed by their thermal expansion and contraction.

20 is a metal cap which serves to hold the terminal rod 12 through the medium of a slide packing 21 and permits thermal expansion and contraction of the long porcelain bushing 1.

As above-mentioned the spring 18 gives a tensile force X 5,300 210 kgn/crn.

to the insulating cylinder 6 and a compressive force to the long porcelain bushing 1 so that the external force such as wind pressure, earthquake, etc., is mostly offset. This remarkably improves the resistance of the long porcelain bushing 1 to bending load and prevents its break by the external force.

When the tensile force of 110 ton is applied through the spring 18 to the insulating cylinder 6, the compressive force of 110 ton is always applied to the long porcelain bushing 1.

When the sectional area of the long porcelain bushing 1 is 1,000 cm. the compressive stress per unit area thereof becomes 110 kg./cm.

Thus, the tensile stress corresponding to the static load in the above Formula (2) can be oflet by the compressive stress applied to the long porcelain bushing 1. It is a matter of course that the greater the tensile stress of the insulating cylinder 6, the higher the mechanical strength of the long porcelain bushing 1.

As seen from the above, since the insulating cylinder 6 fitted into the long porcelain bushing 1 has a breaking pressure suflicient to withstand the internal pressure of 100200 kg./cm. at the time of transient pressure rise, it prevents excessive internal pressure from applying to the long porcelain bushing 1. Thus, the embodiment shown in FIG. 2 ensures a safe protection of the long porcelain bushing 1 against breakage due to the internal and external forces and further provides the important advantage that the long porcelain bushing having a small thickness can practically be utilized, and that a long porcelain bushing for extra-high voltage use can be manufactured in a simple and less expensive manner Without requiring any elaborate apparatus for manufacturing a long porcelain bushing such as a kiln, etc.

The invention has the important feature that the slide seal arranged along the terminal rod serves to permit thermal expansion and contraction of the insulating cylinder. Thus the insulating cylinder must be provided with the flange made integral therewith and secured to the metal fitting and the slide packing must be inserted between said metal fitting and the terminal rod. In other words, the important constructional feature of the invention is that the insulating cylinder is provided with the flange.

Since the fibre-reinforced plastic insulating cylinder is manufactured by winding glass or plastic fibre in longi tudinal direction in order to increase its tensile strength, it is diflicult to produce the insulating cylinder integral with flanges in a single process.

Thus, several layers of glass cloth impregnated with synthetic resin are transversely wound by hand around the end portion of the insulating cylinder which has been completed and then these layers of glass cloth are hardened to form a flange.

Since the insulating cylinder manufacture and the flange forming are done in two separate processes, voids inevitably develop between the flanges and the insulating cylinder and also in the flanges themselves because they are made by hand. Moreover, there are'always produced gaps in those portions of the flange which are provided with threaded holes for screwing bolts in to secure the metal fitting to the flange.

The above-mentioned voids and gaps produced in the flange cause partial discharge therein When the long porcelain bushing is practically used for the extra-high voltage transmission line.

Thus, it is necessary to prevent such partial discharge. FIG. 3 shows a third embodiment of the invention to satisfy the above-mentioned requirement, wherein the outer peripheral surface of the flange 7 of the insulating cylinder 6 is coated with electric conductive or semi-conductive paint 22 whose shielding effect is capable of increasing the partial discharge inception voltage.

In our experiment a fibre-reinforced plastic insulating cylinder, 15 mm. in thickness and 168 mm. inner diameter,

provided with a flange, 250 mm. in outer diameter and 140 mm. in length, was fitted into a long porcelain tube having an inner diameter of 350 mm. The partial discharge inception voltage measured on the insulating cylinder with flange coated with the electric conductive or semi-conductive paint was about twice as large as that measured on the insulating cylinder with the flange not coated with the electric conductive or semi-conductive paint.

In the above experiment an electric conductive paint consisting of silver paint having resistivity of IO- Q-cm. and of a carbon paint having resistivity of lo -10 0mm. was coated in a thickness of about -200 If the inner wall of the threaded hole 23 formed in the flange 7 is also coated with an electric conductive paint 24 to fill up the gap 26 between the threaded hole 23 and the bolt 25 screwed therein, this further increases the partial discharge inception voltage in said gap 26.

The above-mentioned embodiments make use of an insulating cylinder having a uniform inner diameter.

With such insulating cylinder, the long porcelain bushing cannot be made smaller in inner diameter because of the presence of the flanged portion; nor is it possible to reduce the inner diameter of the insulating cylinder because of the presence of the cable terminal cone whose lower end portion has a large diameter.

The cable terminal cone is tapered towards the terminal rod from the cable side. The fourth embodiment of the reinforced long porcelain bushing according to the invention shown in FIG. 4 aims at giving a taper also to the insulating cylinder so that it comes in parallel with the tapered cable terminal cone. That is, in the fourth embodiment shown in FIG. 4 an insulating cylinder 27 made of fibre-reinforced plastic is tapered in such a manner that the outer diameter D at its upper end is made smaller than the outer diameter D at its lower end and a flange 29 made integral with the lower end of the insulating cylinder 27 is located below the long porcelain bushing 1.

30 is a lower metal fitting adapted to secure the long porcelain bushing 1 with a metal fitting 31 and directly secure the lower flange 29 of the insulating cylinder 27 with a metal fitting 32.

33 and 34 show slide packings.

The above-mentioned embodiment provides the advantage that the tapered insulating cylinder 27 has increased breaking strength and that the lower flange 29 of the insulating cylinder 27 located below the long porcelain bushing 1 renders it possible to make the inner diameter D, of the long porcelain bushing 1 smaller than the outer diameter D; of the lower flange 29.

As seen from the above, the reinforced long porcelain bushing according to the invention has a number of excellent advantages and can effectively be applied to the extra-high voltage transmission line.

While the invention has been described with reference to the embodiments applied to the long porcelain bushing for use in the cable terminal cone, the invention may also be applied to porcelain bushings for use in transformers and other electrical machines and apparatus with the same advantages.

What is claimed is:

1. A reinforced long porcelain bushing in which a fibrereinforced plastic insulating cylinder having flanges of the same material at upper and lower ends is provided in a long porcelain bushing hermetically connected to upper and lower metal fittings, in such a manner that the upper flange is fixed hermetically with gasket to upper metal fitting for the insulating cylinder, through which a terminal rod passes, and the lower flange to the lower metal fitting on which the lower end of the porcelain bushingis fixed; said upper metal fitting for the insulating cylinder is fitted to the terminal rod hermetically with a slide packing in between, which slides along the rod according to thermal expansion and contraction of the insulating cylinder; and said upper metal fitting on which the upper 7 end of the long porcelain bushing rests is fitted to the terminal rod which passes therethrough, hermetically with another slide packing which slides along the rod according to thermal expansion and contraction of the long porcelain bushing.

2. A reinforced long porcelain bushing as claimed in claim 1, in which (a) the upper metal fitting for the porcelain bushing consists of two separable, upper and lower, parts fitted hermetically together, the lower part being hermetically attached to the upper end of the long porcelain bushing and the upper part being hermetically fitted to the terminal rod passing therethrough, with a slide packing in between, which permits thermal expansion and contraction of the long porcelain bushing, and (b) a bolt, passing through the upper metal fitting for the long porcelain bushing and having a spring between said upper metal fitting and a clamping member such as nut fitted in threaded engagement to the bolt, is screwed into the upper metal fitting for the insulating cylinder so that compression of the spring by turn of the clamping member causes a tensile stress to the insulating cylinder and therefore a compressive stress to the long porcelain bushing.

3. A reinforced long porcelain bushing as claimed in claim 1 in which said insulating cylinder is a tapered a References Cited UNITED STATES PATENTS 1,988,824 1/1935 Austin.

2,306,186 12/1942 Rankin.

2,431,839 12/1947 Stark.

3,178,505 4/1965 Van Sickle 17431 3,236,982 2/1966 Gonek et a1. 174-30 X 3,242,251 3/1966 Floessel 17431 X 3,325,584 6/1967 Herzig 17414O X LARAMIE E. ASKIN, Primary Examiner US. Cl. X.R. 174-31, 142 

